Seals of the aforedescribed type having been used heretofore in flange connections, pipe joints and the like and may, as described in WO 93/07407 corresponding to EP 0606281, have a central V-shaped configuration in the steel strip and a soft sealing strip between the turns of the steel strip.
The soft sealing strip can project beyond the steel strip. The material of the soft sealing strip is selected to match the conditions under which the seal is to be used. For example, when the seal is to operate at high temperatures, for instance, in the range of 400.degree. C. to 500.degree. C., the soft sealing material can be sealing graphite. At lower temperatures the material can be polytetrafluoroethylene
The flange connections for which such seals may be used are generally standardized connections between pipes and the like or between vessels such as between exchangers, boilers, chemical processing equipment and pipes. Not only are the dimensions of such flange connections standardized but their fabrication tolerances are largely constant.
The sealing force in a flange connection is generated by tightening the bolts mentioned previously. The nominal dimensions of the sealing rings and the flange connections with which they are used can conform to the German Standards DN 10 to DN 2000 and, of course, corresponding United States dimensions and standards. The thickness of the sealing ring in general should amount to 3.5 to 6.5 mm and it has been common practice where a steel spiral is used in or as part of the seal, to provide a number of inner turns of the steel spiral and a number of outer turns of the steel spiral free from intervening soft sealing turns. In these outer and inner regions, the turns of the steel strip can be welded together.
Although seals of the aforedescribed type have been found to be highly satisfactory for most purposes, fabrication problems have arisen. For example, when the seal is fabricated as described in the aforementioned publication, the soft seal cushion between the steel strip and the juxtaposed faces of the flanges is formed exclusively by the projecting portions of the turns of the soft sealing material, which, for this purpose, must extend by a width of at least 1 mm beyond the steel strip on each side. The cushions of the soft sealing material which are thus formed serve to prevent direct contact of the steel strip with the juxtaposed faces of the flange or at least prevent detrimental contact of the spirally wound steel strip with the flange faces, the projecting portions of the soft material deforming for this purpose upon tightening of the flange coupling. Because it is necessary to carefully position the seal and carefully tighten the flange coupling to uniformly form the cushions as described, the assembly process is time consuming and thus expensive.
Seals utilizing the principles described are generally mass produced by shaping the coil of the steel strip on a V-shaped forming periphery of a wheel and by winding the soft sealing strip into the turns of the steel strip on the wheel as well. On one edge of this wheel a flange is formed and on an opposite edge a flange forming disk cooperates with the coiling wheel. A recess may be provided at the middle of the wheel periphery to accommodate the V-shaped portion of the strip. The depth of the recess can correspond to a radial dimension of the sealing ring and the width of the recess to the width of the soft steel strip. Since the steel strip and the soft seal strip may be simultaneously wound in this groove, there occasionally is a relative shift of the steel strip and the soft sealing strip that can detrimentally influence the sealing ring which results.
In general, the ring must be fabricated with very narrow tolerances to satisfy the requirements for effective flange sealing and the conventional process in many cases was not able to satisfy the need for such narrow tolerances.
While experience has shown that it is possible to wind interleaved turns of the steel strip and soft sealing strip reproducibly when the latter does not project more than 0.5 mm beyond the steel strip and where any projection beyond the steel strip is preferably 0.2 to 0.4 mm, with high tolerances, the projection of the soft sealing strip in excess of 0.5 mm beyond the steel strip results in shifts as described which tend to be additive from turn to turn. In many cases, therefore, the conventional seals did not possess adequate sealing function when fabricated in this manner or the sealing effects were not reproducible. To obtain reproducible seals of the configurations hitherto used, it was necessary, therefore, to resort to much more expensive fabrication techniques.